Antenna systems of the type mentioned above are previously known from U.S. Pat. No. 5,231,407 and WO-A1-91/01048. One advantage of the separation between transmitter and receiver is that the requirements for duplexing filters will decrease. However, a problem is the coupling between the transmitting and the receiving antennas. To decrease said coupling the antennas used in U.S. Pat. No. 5,231,407 are tunable narrow band antennas, while the antennas described in WO 91/01048 are arranged at different ends of the telephone.
At first sight, the use of more than one antenna can be seen as waste of space etc., but nevertheless, a number of inventors have pointed out numerous advantages. The present invention can be said to be a new and inventive way to utilise the concept of two or more antennas or antenna functions for operation in respect of a single system transmitting/receiving band, to save space and decrease losses in human tissue. Some further examples of the use of more than one antenna that are known, used for achieving diversity or directional properties, for minimising the influence of a users hand, and for satellite telephones.
To achieve diversity, more than one receiving antenna is used together with one transmitting antenna (usually the same as one of the receiving antennas). 5-10 dB fading reduction is reported as a result of the use of diversity reception. EP-B1-0 214 806 and EP-A1-0 648 023 disclose two examples thereof. A further example is shown in WO-A1-95/04386. Diversity is standard in the Japanese PDC system and typically one whip antenna combined with one PIFA (Planar Inverted F Antenna) are used.
Directional properties have been suggested in order to improve antenna gain in the direction of the base station (i.e. in a variable way) and to suppress interfering sources. EP-A1-0 649 227 is one example.
EP-A1-0 752 735 discloses the use of multiple antennas in order to minimise the influence of the users hand, simply by using one of the antenna elements which are not covered by the hand (as detected by the VSWR).
Satellite telephones generally have strong requirements on the antennas, such as big difference between transmitting and reception frequencies or extreme requirements on low losses (i.e. filters should be avoided). WO-A1-97/26713 and WO-A1-98/18175 are two examples hereof, where separate transmitting and receiving antennas with the same circular polarisation are used.
Modern mobile phones are small and thus the interaction between antenna, phone body and user will become more important than earlier. There is also normally a requirement for two or more frequency bands and a recent trend is to integrate the antenna function into the telephone body making it invisible from the outside, which is customary named built-in antenna. According to the present invention, a number of benefits can be achieved by using separate antennas for transmitting and receiving if they are implemented according to special principles of the present invention, which will be described below. The requirements for transmitting and receiving antennas are quite different and with the diminishing size it becomes more and more important to optimise each of them separately. It is well known that antenna performance will go down when the antenna is made smaller.
Since the mobile telephones today are very small and the antennas, during telephone calls, will be located close to the head of a user, much attention is paid to the effects on the human body when exposed to electric fields. An issue especially discussed is the SAR (Specific Absorption Rate) values, which preferably should be low. In the documents mentioned above, no efforts are shown how to decrease the SAR values.
SAR (Specific Absorption Rate) is used to quantify electromagnetic fields in respect of influence to the human body, and is also applicable in the near field. SAR is defined as the power loss per a certain unit of body tissue, and for instance FCC (Federal Communications Commission) in the US requires less than 1.6 mW per gram. The phone systems require a certain power level (such as 2 W peak and 0.25 W average for GSM in highest power level). It should however be noted that, the field near the antenna can be different for different types of antennas, even if the field far from the antenna should be the same. SAR is measured inside a dummy head, or can be calculated. Due to SAR's nature of power density, a smaller antenna structure carrying the same power as a bigger structure is more likely to be close to the limit value. This is the case for most phones using small antennas. The general development of the phones thus calls for SAR optimised solutions. Bigger antenna structures will generally cause lower SAR values, but modern telephone design requirement do not support increasing size. Antenna efficiency is another important characteristic and efficiency and SAR are somewhat correlated as high SAR obviously means extra losses. The term SAR will be used herein when reference to existing limits (stated by FCC, CENELEC etc.) or corresponding measuring methods is relevant but otherwise the more general expression "losses in human tissue" will be used.
To define some terms reference is made to FIG. 1a, which shows a typical telephone with a helical antenna, which is one of the most common types of antennas today. The user 1 holds the telephone body 2, provided with an antenna 3, to the ear 4. The radiated power Prad has to comply with the requirement of the telephone system in question. Prad is smaller than the power Pin fed by the transmitter, and the quotient between them gives the efficiency. A part of the loss in the human tissue (head, hand, etc.), causes a (very small) heating 5 of the human tissue close to the antenna, and many times more heating occurs at locations as 6 along the phone. For the subsequent discussion it should be noted that the telephone configuration in FIG. 1a can be understood as a very asymmetric electric dipole as shown in FIG. 1b. The asymmetric dipole 1b differs from the common symmetric dipole in FIG. 1c only by its feeding impedance. The currents along the dipoles 1b and 1c are the same, which is the reason for the occurrence of the current and loss maximum at 5 in FIG. 1a.
For a transmitting antenna, both SAR and efficiency are important. For losses in human tissue it can be shown that various small antennas radiating the same power and located on the same distance from the ear can give values differing more than 100 times. Should much lower SAR values be required than those that can be achieved by the typical antenna of today, it will be necessary to use some of the more efficient antenna principles with regard to the losses in human tissue. Naturally magnetic type antennas (loops etc.) will give less SAR in their near field as compared to antennas of the electric dipole type. This can be exemplified by studying the fields from a electric dipole and a magnetic dipole, radiating the same power. When r decreases, the electric fields are increasing as 1/r.sup.3 and 1/r.sup.2, respectively, and thus the magnetic dipole (1/r.sup.2) will have much lower E-field (corresponding to SAR) at very small distances, in spite of the same field at long distances.
The fields of the electric dipole are illustrated very schematically in FIG. 2a where a simple linear antenna 10, typically half a wavelength or smaller, in total length, is fed over its symmetric gap 11 by a feeding line 12. The dipole is directed along the z-axis in a thought co-ordinate system where the electric 13 and magnetic 14 field can be described by the following equations expressed in standard spherical co-ordinates r, .theta. and .phi.: ##EQU1##
Where: PA1 k=wave number (=2.pi./.lambda.), PA1 Z.sub.0 =377 .OMEGA., PA1 I=current, PA1 l=effective length.
FIG. 3a shows the corresponding fields around a magnetic dipole exemplified by a small ring 16 fed with current from a line 17. Its corresponding electric 18 and magnetic 19 fields are similar to those of the electric dipole. With suitable scaling they are in fact identical if electric E and magnetic H fields are exchanged and scaled. The mathematical expressions are: ##EQU2##
Where, further: A=area of the loop.
One important property obvious from those equations is that the far fields (radiation fields) for increased distance r decays as 1/r while the radiated power is preserved. Close to the dipole the variation with distance is 1/r.sup.2 or 1/r.sup.3, and this is illustrated by FIGS. 2b (electric dipole) and 3b (magnetic dipole). Close to the dipole the radial field is strongest, and the radial field is electric for the electric dipole and magnetic for the magnetic dipole. The radial fields disappears far away from the dipole. Losses in human tissue are depending on the electric field a human body is exposed to, and since the losses occur very close to the radiating structure, an electric dipole will have very different SAR properties compared to those of a magnetic dipole.
The field at a distance d.sub.n very near a dipole (d.sub.n &lt;.lambda./10) is quite different from the radiation field at a distance d.sub.f far away from the dipole (d.sub.f &gt;.lambda./2). SAR is depending on the near field only, while the radiation is depending on the far field only. It is an interesting fact that different antennas having the same radiated power may have very different near field. One of the really efficient way to reduce losses in human tissue is thus to choose the proper antenna element rather than reducing both far field and near field, which is done by various non-approved attenuating products on the market said to "screen" the radiation. Most modern cellular systems will try to increase output power to maintain the radio connection causing shorter battery lifetime and less reception sensitivity but generally not a relatively decreased nearfield.
It can also be expected that antenna structures isolated from the phone body (by distance or symmetry) would have less losses as many phones show maximum loss per unit of volume somewhere along the phone body, due to the currents along the same.
One SAR measurement of a magnetic dipole structure is given in: "Miniature dielectric loaded personal antenna with low user exposure", Leisten et. al., Electronics letters, Aug. 20, 1998.
It is well known that the size of an antenna is critical for its performance, (see Johnsson, Antenna Engineering Handbook, McGrawHill 1993, chapter 6) which can be expressed as a limitation of the product of the relative bandwidth (.DELTA.f/f) and the efficiency (.eta.), which always is smaller than a constant multiplied by the efficient volume (V) of the antenna (as expressed in cubic wavelengths): EQU (.DELTA.f/f).eta.&lt;constant (V/.lambda..sup.3)
The constant has been suggested to be close to 13, but in many cases it is far from obvious to determine the "effective volume of an antenna", since it may include a portion or a quite large portion of the exterior structure (typically the whole) of the telephone body. Because of this, the equation generally can not be used for accurate calculations, but rather to predict an approximate size. The size predicted by this equation apply for an antenna in the 900 MHz band, comparable to the whole phone body, and the typical antenna in that band does indeed engage the whole telephone to support the currents creating the radiation. Due to its size the typical phone antenna of today for GSM, AMPS etc. is thus rather a coupling structure to the phone body itself which at 900 MHz is a crude approximation of a .lambda./2-dipole antenna. For clarification, when the word antenna is used in the following, it relates to the whole part that participates in the radiation. Antenna element is that part (e.g. a helical element, PIFA etc.) which is fed via a feed portion. The typical mobile phone antenna used today consists of the conducive portion of the phone (circuit board, screening structures and perhaps conductive housing) fed by the antenna element. The same antenna element can be included in plural antenna functions, when fed in different feeding modes. The current on the phone body is generally a significant contribution not only to the radiation but also to the SAR. As a consequence of this volume condition, an antenna comprising a small antenna element, which is isolated from the phone body will have a small volume compared to the phone body, and is thus also probably a rather poor antenna in terms of efficiency and bandwidth, if it is necessary to cover the full GSM-band. The term "small supporting structure" will be used subsequently herein about rather small structures, typically having a greatest measure of one wavelength or smaller, which are supporting an antenna element of the same or smaller size. One important property when designing mobile phone antennas in contrast to antennas mounted on big structures (towers, vehicles etc) is that the mobile phone must be able to operate by itself and antenna pattern, antenna impedance and other characteristics will be heavily influenced by the limited size of the structure. This will be different for different antennas but antennas intended to be mounted on a ground plane (such as a monopole or slot on a ground plane) will have a very different radiation pattern if the ground plane is just one or two wavelengths large as compared to the case when the same antenna is mounted on an "infinite ground plane" which can be understood as several wavelengths big. For the common helical antenna (normal helix) on a mobile phone it can be verified that while its radiating impedance on a large ground plane may be 2-3 ohms the impedance when installed on a mobile phone typically have increased to 15-20 ohms. This will change the conditions significantly for the function and design of the antenna, for instance in terms of bandwidth. Because of this drastic differences it is most cases necessary to distinguish between the function of antennas mounted on "a large structure" and antennas mounted on "a small structure" but obviously this distinction is only necessary when the antenna itself is a "small structure". The term "small supporting structure" will be used to characterise these cases. The chapter 6 (by Wheeler) in "Antenna Engineering Handbook" referenced above describes "small antennas" in the meaning that they can be enclosed in a sphere having one wavelength or less as circumference ("radiansphere"). On a phone this generally applies to the antenna element itself but in most cases not to the whole phone. Wheelers' term "small antennas" or "radiansphere" should thus not be confused with the term "small supporting structure" used herein.
For a receiving antenna the interaction with the user does not create any SAR problem. On the contrary, the efficient volume of the antenna can be increased by the presence of a user. Interaction with the user may thus even be favourable. For sensitivity purpose, a second receiving antenna can be included to implement diversity function. This can be done by adding a separate antenna, or in some cases by including a second receiving antenna in the transmitting antenna.
There will also be a change of the coupling when the phone is gripped by the hand of a user. Different specific designs of individual antenna elements can have very different degradations. It should be recalled that most present phone antennas actually are coupling elements to the body of the phone which is radiating by carrying currents along its length. This is generally independent of the appearance or type of the antennas.
Nearly all modern mobile phones can be described as electric dipoles directed along the phone which for simplicity is named "vertical" below. From the observations above this implies relatively high SAR and decreased radiation efficiency. The antenna is here the antenna element plus at least a part of the phone body and the far field radiating function have essentially the same radiation characteristics regardless of the antenna element being a helix in the top of the phone, a PIFA on its back or side, a slot antenna on its back etc. In this group are also included short extendible whip antenna elements which using the terminology herein constitutes one antenna but which can be mechanically modified to improve some properties. In a pictorial way for the receiving mode, a part of the electric field around the phone is "attracted" by the antenna element (helix, PIFA etc.), so that a portion of the displacement current of the electric field enters the antenna element.
Telephones having external antennas which are or can be directed more or less perpendicular to the head are known. FIG. 4 shows one example according to EP-A1-0806809 having an antenna 52, which can be bent. By the bending and the length of the whip antenna, the radiation will be, to a rather large extent, related to an electric dipole perpendicular to the skin. This may be expected to increase the efficiency.
Magnetic dipoles in the shape of a ferrite core have been used in paging systems in the HF to lower VHF frequency range. They are typically attached near the waist or placed in a pocket and thus parallel to the local surface of the body.
FIG. 5 shows an example of this, with the pager 53 attached adjacent to the waist 54 of a user, and fitted with a ferrite core 55 acting as a magnetic dipole. Ferrites have so far been quite poor at the frequencies used as mobile phone frequencies, otherwise this method would improve the magnetic dipole. Their efficiency is greatly increased by the presence of the user. These antennas are only used as receiving antennas, and not as transmitting antennas.
Depending on field and polarisation some antennas will have improved function close to the user, while other will have degraded performance. Most modern antennas belong to the second group. The above mentioned pager antenna and the antenna disclosed in EP-A1-0806809 belong to the first group. With the simplifying assumption that a telephone is shaped like a box, the division of phone antennas into six types (two kinds of dipoles times three perpendicular geometrical orientations) is useful to characterise their radiation properties, and their type of interaction with the user.
The reason for the very common use of an antenna combination, such as a vertical electric dipole transmitting antenna and a vertical electric dipole receiving antenna, which has some less favourable properties as mentioned above, is probably the difficulty to obtain efficiency and bandwidth within the small space available within the phone. The easiest way to obtain radiation efficiency and bandwidth in free space measurements is to use the length of the phone (typically around .lambda./2 at GSM/AMPS). The "expense" is a relatively high SAR and a considerable reduction of the efficiency when the phone is moved from "free space" position to "talk position". For a typical mobile phone the efficiency, in practical use, is about 10% as compared to an ideal case (.lambda./2-dipole in free space). This figure can be readily improved by using an antenna element giving less degrading interference with the user. One conclusion from this is that the telephone preferably should be optimised for talk position rather than for free space. One important part of the invention is to avoid the destructive interference with a user. Furthermore the SAR is normally close to the upper limit allowed by for instance FCC in USA. It should be observed that the statements herein about the overall electric dipole function applies to small antennas only (fixed helices or "built-in" antennas). For instance an extendible antenna of essentially half-wavelength typically has low losses in human tissue and corresponding high efficiency due to its isolated function relative to the body of the phone. Phones of regular size for operation at higher frequencies (1700-1900 MHz) are "bigger" as expressed in wavelengths, generally improving the size-bandwidth-efficiency trade-off situation.